If upon entering the summoning chamber you get the message stating that there are to few players in the instance and it will close (this year it hasn't been closing and the fight continues even though you get the message) there is no need to abandon all hope, it is still possible to complete this event with as few as 2 people (assuming the 2 are strong dps and survivable) though more are always better.
First I have to clarify, this requires that at least one person has mass invisibility, scrolls will work, especially if you are an artificer and thus get longer then a 13 minute duration... an extended mass invis from an arcane caster is preferred though...
you should have 10 minutes to prepare for the dragons arrival.
step one - click on each altar and then cast mass invis on the altar, then everyone leave the room (do this for all 4 altars, do not leave a single person in any of the summoning rooms)
step two - wait for the dragon to spawn, and go into the room he spawned in, leaving 1 or 2 people to flip the lever (if you only have 2, leave 1 person on the lever the other in the room with the dragon).
note - the dragon will only move to rooms that have players in them after he initially spawns, therefore he will stay in the same room the entire time as long as it is the only room that has any players in it...
step three - kill the dragon, you will have as long as the mass invis lasts on the altars to kill the dragon. this is why an extended mass invis from an arcane caster is the best option.
I have been in a couple of spectral dragon fights where we have had to employ this technique, one was a success, the other was a failure. the failure lasted about 12 minute after the dragon spawned, and the success lasted about 4 or 5 minutes after the dragon spawned... i was on an artificer and was able to mass invis the altars using scrolls.
In the failed run there was only 1 other person (a monk) in the instance with me. the monk was on lever duty, and I was in the room with the dragon. the monk was unable to keep the lever flipped and the light on. so my dps on the dragon was very low. we lasted until the mass invis ran out on one of the altars and it got destroyed.
The time we succeeded with this technique there were 5 of us in the instance. one was a wizard with mass invis, he went to each of the altars and invis'd them. and then was one of the 2 people on lever duty, the other 3 of us (myself - arti, a cleric, and an assassin rogue) went into the room with the dragon and killed him. the 2 people on the lever were able to keep it open nearly the entire time and we were able to kill the dragon with no trouble at all. the dragon never left the room it spawned in and we prioritized killing the gorgoyles in the room with us over dpsing the dragon.
The hardest part is getting people to not get stuck in the mindset of "we have to defend every altar or we fail quickly", as long as the altars are invisible and there is no one in the room, the gargoyles do not attack the altar.